The primary objective of the proposed research is to determine and examine those regions in the cat and monkey thalamus that receive converging projections from the dorsal column nuclei, the lateral cervical nucleus and the lateral spinothalamic tract in the spinal cord, as well as from the first and second somatosensory areas in the cerebral cortex. This research will be effected by means of a differential labeling strategy that permits the direct observation of pathways converging on a single neural region. The strategy uses degeneration and autoradiographic tracing methods simultaneously in a single experiment to differentially label the terminals of two different pathways. A second objective of the proposed research is to use this strategy to help define the neuroanatomical borders of nuclei within the thalamus that are divisible on functional grounds. Hence, autoradiographic methods will be used to label known sources of input to one functional nuclear region, while degeneration methods are used simultaneously to label a known source of input to the other, adjacent functional region. Borders should become evident where autoradiographically revealed terminals end and degeneration-revealed terminals begin. It is hoped that these experiments will provide evidence that helps relate structure with function in attempts to decipher those central nervous system mechanisms that are involved in somatosensation. BIBLIOGRAPHIC REFERENCES: Mash, D.C., Worden, I.G., and Berkley, K.J. Projections of the lateral cervical nucleus and the dorsal column nuclei to regions adjacent to and within the n. ventralis posterolateralis of the cat thalamus. Neuroscience Abstracts, 1976, in press. Berkley, K.J. Projections to the intralaminar group complex of nuclei in the cat thalamus. Neuroscience Abstracts, 1976, in press.